The present invention relates to apparatus for driving casing while permitting access to the casing interior, and particularly concerns improved structural arrangement of such hammer.
Our prior application, Ser. No. 888,312, describes a method and apparatus for drilling wells, particularly water wells, in which the casing is driven at the same time the well is being drilled, with fluid forced downwardly through the hollow drill string to remove loose material. The casing hammer provides an elongated annular chamber in which is mounted a piston that is pneumatically reciprocated at a relatively high repetition rate to impact upon an annular anvil captured in the hammer at the lower end of the annular chamber and transmitting piston blows to an annular driving head that rests atop a driven casing section. The annular chamber of the hammer of our prior application is formed by an outer sleeve that is held in position to and between a rear end cap and a front housing by a number of tension rods extending longitudinally of the hammer. Similarly, the inner sleeve is held to and between the end cap and a front retainer ring by a number of longitudinally extending tension rods. Debris is forced upwardly by the fluid passing downwardly through the drill string, and flows upwardly around the drill string between the drill string and the inside of the hammer. A seal at the front (lower) end of the hammer is provided to prevent such debris from entering the interior of the rear portion of the hammer, within the inner sleeve of the annular chamber. It has been found, however, that this seal is often at least partially ineffective, and thus debris and mud frequently enters the interior of the inner sleeve between the drill string and the sleeve and becomes entrapped in the inner sleeve tension rods. Such debris often fills the interior space so that rotation of the drill string tends either to rotate the inner sleeve or to twist the inner sleeve tension rods. The seal between the rotating drill string and the interior of the forward end of the hammer may wear and as it wears the leakage of debris and mud increases to aggravate this problem.
The hammer described in our prior application is constructed so that the entire weight of the hammer when it is resting upon the upper end of a casing to be driven, is transmitted through the inner sleeve of the annular chamber and thence via the anvil and driving head to the casing. This places undesirable and unnecessary strain upon the inner sleeve.
Accordingly, it is an object of the present invention to provide a pipe hammer that eliminates or minimizes above-mentioned disadvantages.